The invention generally relates to polymers formed from aromatic acids. More particularly, the invention relates to aromatic acid monomers which contain small amounts of materials that can provide unexpected advantages during the polymerization or copolymerization of those acid monomers, as well as to processes for manufacturing such aromatic acid monomers and polymers.
The manufacture of aromatic acids useful as monomers typically is a complex, multistep process. For example, 2,6-naphthalenedicarboxylic acid (2,6-NDA) can be manufactured by a five step synthesis process which includes the steps of reacting o-xylene and butadiene in an alkenylation reaction to produce 5-ortho-tolylpentene, cyclizing the 5-ortho-tolylpentene to form 1,5-dimethyltetralin (1,5-DMT), dehydrogenating the 1,5-DMT to produce 1,5-dimethylnaphthalene (1,5-DMN), isomerizing the 1,5-DMN to produce 2,6-dimethylnaphthalene (2,6-DMN), and oxidizing the 2,6-DMN to produce 2,6-NDA.
Crude NDA produced by such a process will contain a wide variety of what are believed to be undesired process-related materials. Many of these materials will be isomers of 2,6-NDA or mono- or trifunctional reaction products. Other undesired process-related materials contained in the crude NDA will be reagents such as catalyst metals carried through the various reactions steps, and color bodies formed during the reaction steps. As used herein, the term xe2x80x9cprocess-related materialxe2x80x9d means any material that is formed or added in any process step leading up to the manufacture of aromatic acid monomer product, including but not limited to, catalysts, products of side reactions, undesired oxidation products, undesired isomers and the like.
It is believed that in the preparation of polyesters from monomers such as NDA, monomer purity is critical to satisfactorily achieving high molecular weight polymers and a sufficiently fast kinetic rate of polymerization. For this reason, polymer manufacturers typically require that monomer impurities such as mono-functional and tri-functional glycols and carboxylic acids be minimized or eliminated from monomers to be used in polymerization reactions. For example, terephthalic acid and isophthalic acid typically are expected to contain less than 200 parts per million or less by weight total of monocarboxylic and tricarboxylic acids. Similarly, ethylene glycol used in polymerization reactions typically is expected to contain no detectable impurities.
Tricarboxylic acids are thought to be undesirable because such trifunctional compounds can cause undesired cross-linking of polymer chains. Such cross-linking is reported to contribute to slow rates of crystallization and polymer brittleness, both of which are undesired characteristics in many applications. Additionally, when cross-linking becomes substantial, a xe2x80x9cgel pointxe2x80x9d is reached. At this point, the polymer cannot be melt polymerized or melt fabricated and is no longer considered to be a thermoplastic material.
Monocarboxylic acids and other monofunctional materials are believed to be undesirable components in monomers because they act as xe2x80x9cchain-stoppersxe2x80x9d which inhibit the development of molecular weight and because they decrease reaction kinetics. If the concentration of such materials is too high, the polymerization rate can become zero due to termination of otherwise reactive end-groups.
Color bodies of various types are thought to be undesirable in monomers. The presence of color bodies in monomer can result in substantially greater color in a polymer than would appear likely from seemingly small amounts of color visible in a monomer, thus making even minute amounts of color bodies in monomers undesirable. As used herein, the term xe2x80x9ccolor bodiesxe2x80x9d refers to any carboxylic acid containing process-related material present in a monomer or polymer that can contribute to the presence of color in the monomer or polymer if present in sufficient amount.
Metals such as entrained catalyst metals also are thought to be undesirable components in monomers. For example, entrained cobalt and manganese oxidation catalyst are believed to be undesirable monomer impurities because it is expected that they may affect the rate of polymerization and polymer color in an unpredictable way. Such metals also are thought to sometimes affect the amount of color visible in a monomer or polymer.
Because it is believed that the presence in monomer of undesired process-related materials such as byproducts, reagents and impurities like color bodies can result in an inferior polymer product, substantial effort typically is devoted to improving the purity of monomers such as 2,6 NDA to provide a quality of product deemed acceptable by customers.
For example, purified aromatic acids have been produced from crude aromatic acids by slurrying the effluent from a crude aromatic acid oxidation process, passing the slurry through a plurality of heaters until the reaction products are dissolved, passing the resulting solution over a purification catalyst, and thereafter crystallizing a purified product. Such a process requires substantial time and energy beyond that expended to produce crude aromatic acid, and therefore substantially increases the cost of the monomer.
Alternatively, high purity monomer can be manufactured by starting with a relatively high purity feedstock, such as a process in which relatively pure 2,6-naphthalenedicarboxylate (2,6-NDC) is hydrolyzed to form relatively pure NDA. This process also is cost intensive because of the complexity and expense of producing the relatively pure NDC feedstock.
What is needed is a cost effective way to produce aromatic acids such as NDA which are suitable for use in polymer applications.
Surprisingly, we have found that the presence of certain levels of process-related materials in aromatic acid monomers can result in monomers that perform as well as or better than higher purity aromatic acid monomers when used in many polymer applications.
In some applications, the presence of certain levels of catalyst metals can result in more rapid polycondensation and solid state polymerization reactions, thereby improving the economics of these polymerization reactions without affecting the desired properties of the polymer product.
In other applications, the presence of certain trifunctional materials in the aromatic acid monomer product provide for branching of polymer chains, thereby providing increased melt strength which is useful when molding articles from the polymer.
In still other applications, the presence of certain levels of metallic impurities and color bodies provides for an aromatic acid monomer that has a brownish cast that is useful in particular end uses, including but not limited to the packaging of drinks such as beer in brown polymer bottles.
While in some cases the foregoing aromatic acid monomers might be produced directly as solids separated from the product of an oxidation reaction, typically aromatic monomer product in accordance with the present invention will be produced by relatively simple post-processing of oxidized aromatic feedstocks, such as by slurrying or washing crude aromatic acid in an appropriate solvent under the appropriate process conditions. Monomer product manufactured in this way can be both less expensive and advantageous in certain end uses.
The following detailed description of preferred embodiments of our invention focuses on the advantages of our invention with respect to the preparation of 2,6 naphthalenedicarboxylic acid monomer product and polymers made therefrom. As will be discussed later in more detail, the advantages of the invention also are believed to be useful in connection with other aromatic acid monomers such as terephthalic acid, isophthalic acid and other isomers of naphthalenedicarboxylic acids.
As noted above, 2,6-naphthalenedicarboxylic acid (2,6-NDA) can be manufactured by a five step synthesis process which includes the steps of reacting o-xylene and butadiene in an alkenylation reaction to produce 5-ortho-tolylpentene, cyclizing the 5-ortho-tolylpentene to form 1,5-dimethyltetralin (1,5-DMT), dehydrogenating the 1,5-DMT to produce 1,5-dimethyinaphthalene (1,5-DMN), isomerizing the 1,5-DMN to produce 2,6-dimethylnaphthalene (2,6-DMN), and oxidizing the 2,6-DMN to produce 2,6-NDA. Aromatic feedstocks such as the 2,6-DMN oxidized in this process preferably contain at least 97 mole percent of the feed material which is to be oxidized to the acid, calculated as a mole percent of all aromatic material in the feedstock.
Crude 2,6-NDA produced by the foregoing process preferably contains at least 93 mole percent acid monomer and typically is expected to contain unacceptable levels of one or more of the following materials: trifunctional materials, 1-bromo-2,6-NDA, 2-naphthoic acid, 6-formyl-2-naphthoic acid, cobalt, manganese, bromine, iron and various color bodies. We have found that it frequently is not harmful, and in many cases it is advantageous, to permit certain levels of metals, tri-functional compounds, and color bodies to be present in 2,6-NDA monomer product used in polymerization reactions. In many cases, these acceptable and advantageous material levels can be obtained by relatively simple processing of the oxidation product of 2,6-DMN, thereby eliminating the need for costly purification steps such as recrystallization.
Acceptable and preferred levels of the foregoing materials consistent with our invention are listed in Table 1, below. The ppm ranges listed refer to ppm by weight of the material present in NDA monomer product.
NDA monomer having one or more of the foregoing materials in concentrations in accordance with our invention readily can be produced, for example, by slurrying crude NDA oxidation product to remove a fraction of such materials, while permitting a desirable, or at least non-deleterious, portion of such materials to remain in the monomer. As used herein, the term xe2x80x9cslurryxe2x80x9d refers to any process which employs a solvent to wash or disperse a crude oxidation product, but specifically excludes any process which dissolves greater than about 10 mole percent of a desired aromatic monomer present in crude oxidation product, such as a recrystallization step. Other examples of xe2x80x9cslurryxe2x80x9d processes in accordance with the invention include the use of higher solvent volumes in the reactor in which the aromatic feedstock is oxidized to render the process-related materials more soluble, thereby somewhat reducing the levels present in the product, adding or increasing the volume of solvent in the crystallizer train of the oxidation process to reduce the presence of process-related material by dilution, and the use of filtration with a solvent wash to reduce the level of process-related materials remaining in the monomer product.
For example, crude 2,6-naphthalenedicarboxylic acid can be recovered directly from 2,6-DMN oxidation mother liquor. The crude 2,6-NDA then can be redispersed or reslurried in a suitable solvent such as water, a low molecular weight carboxylic acid, or a mixture of water and a low molecular weight carboxylic acid at a weight ratio of about 0.1 to about 1 part of 2,6-naphthalene dicarboxylic acid per part of solvent. Preferred process conditions for the reslurry process include temperatures of from 60 to 125xc2x0 C., with 75 to 110xc2x0 C. being most preferred, and pressures of from about 0.5 to 3 atmospheres, with pressures from 1 to 2 atmospheres being most preferred. Solvent acid to water ratios can range from 100 percent acid to 100 percent water, with the preferred acid to water part ratio being from about to 90:10 to 50:50, with the most preferred ranges being about 80 parts acid and 20 parts water.
Preferably, at least a portion of the solvent used to redisperse or reslurry the 2,6-naphthalene dicarboxylic acid in this manner is a process stream or process-derived stream such as condensate from the overhead of the oxidation reaction mixture. In this case, solvent comprising water and an acid such as acetic acid can be returned, at least in part, to the oxidation reactor. Alternatively, the solvent can be distilled to recover the low molecular weight carboxylic acid for recycle to the oxidation reactor. Solvents may contain other process materials that will not substantially affect the slurry process or properties of the monomer product, such as alcohols or acetates generated in the process. Such process streams should, however, contain little or none of the process-related materials sought to be minimized in the slurry process.
The foregoing slurry step provides for a relatively purer 2,6-naphthalenedicarboxylic acid. In many cases, such a 2,6-NDA monomer product in accordance with the invention will be suitable or preferred for certain applications over a monomer product produced from a more complex process having additional purification steps.
After this slurry step, the 2,6-naphthalenedicarboxylic acid can be separated from the solvent by any method or methods known in the art for partitioning a solid from a liquid phase such as, for example, centrifugation, filtration, or settling.
Of particular interest in the reslurried NDA are the concentrations of catalyst metals such as cobalt and manganese, the ratio of cobalt and manganese metals, the level of multifunctional aromatic compounds, and the level of colored impurities.
The levels and ratios of catalytic metals are important both because they will affect the polymerization rate of the monomer and because their presence may, in some cases, influence the final polymer color. For NDA applications, the total amount of Co and Mn present in the reslurried material should be no more than about 10,000 ppm by weight in the reslurried product, with 500 to 2,000 ppm being preferred, and 1000 to 1,500 ppm being most preferred. The molar ratio of Co to Mn can range from 5:1 to 0.2:1, with the preferred ratios being between 4:1 to 0.25:1, and the most preferred ratios being between 3:1 and 0.5:1.
The levels of multifunctional materials are important when the polymer to be produced from the aromatic monomer requires additional melt strength. For NDA applications, trifunctional naphthalenic moieties are the more likely species, with 1,2,6-, 1,3,7- and 2,3,6- naphthalene tricarboxylic acids predominating in the mix. Preferably, these trifunctional species will be present in the reslurried NDA in an amount between 50 and about 10,000 ppm by weight, preferably between about 200 and 9,000 ppm by weight, and most preferably between about 150 and 8,500 ppm by weight. When other aromatic monomers such as PTA are the subject of the invention, trifunctional acids such as 1,2,3-, 1,2,4- and 1,3,5-benzene tricarboxylic acids, and mixtures thereof are the more likely trifunctional species, and may be present in the ranges set forth above for the napthalenic trifunctional species. Mixtures of any and all of the foregoing trifunctional impurities may, of course, be present in accordance with the invention, and impurities having a functionality greater than 3 may also be advantageously utilized in accordance with the invention. As used herein, the term xe2x80x9ctrifunctional materialxe2x80x9d means any process-related material having three functional groups capable of reacting with a glycol monomer under polymerization conditions. The term xe2x80x9cmultifunctional materialxe2x80x9d means any such material with a functionality of three or more.
Polyester color is a very important performance requirement in certain applications, while in other applications, color is not important. Sometimes, a color such as brown is required for certain packaging applications. The brown color typically is achieved by the addition of dyes which usually are high molecular weight organic compounds. Dyes are undesirable because they can detract from the polyester properties, especially barrier permeation to gases such as oxygen and carbon dioxide. Additionally, dyes are expensive, and can be undesirable from environmental and recycling standpoints. Thus, color bodies present in an aromatic acid monomer may be useful for inducing a color such as brown into subsequently formed polymers. Color bodies useful in accordance with the invention include benzcoumarin, pentaquinone, pentacene and flourenone structures containing carboxylic acid functions. Typically, these color bodies should be present in an amount between about 50 and about 500 ppm by weight, more preferably between about 50 and 250 ppm, and most preferably present at a level of about 150 ppm.
Slurried NDA in accordance with the invention also can contain monofunctional impurities including, but not limited to, such aromatic acid impurities as benzoic acid and benzoic acid substituted with groups such as methyl, bromo, and formyl groups, as well as 1- and 2-naphthoic acid and 1- and 2-naphthoic acid substituted with groups such as methyl, bromo, and formyl, and mixtures thereof. The concentration of monocarboxylic acids in a reslurried NDA typically is from about 50 to 5,000 ppm by weight, preferably 100 to 4,000 ppm by weight, and most preferably about 150 to 3500 ppm by weight. As used herein, the term xe2x80x9cmonofunctional materialxe2x80x9d means any process-related material having a single functional group capable of reacting with a glycol monomer under typical polymerization conditions.
Each of the foregoing materials need not be present in the amounts mentioned above if the desired advantage attributable to that material is not required in the desired monomer application.
By way of example, the crude NDA can be reslurried to yield an NDA monomer having the approximate specifications set forth in Table 2, below.